A study was just published on that. BAsically, the reesearcher found that the red dye was there as a sunscreen to protect the leaves after chlorophyll was broken down. The tree uses energy (sugar) to make the dye, in exchance for keeping the leaves alive while nitrogen is salvaged for next year.
The study claimed that in trees that sometimes, but not always show red leaves, it is an indicator of nitrogen stress.
LINK: http://www.sciencedaily.com/releases/2007/10/071025112042.htm
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